Many herbicides injure crop plants at herbicide application rates necessary to control weed growth. Accordingly, many herbicides cannot be used for controlling weeds in the presence of certain crops. Uncontrolled weed growth, however, results in lower crop yield and reduced crop quality inasmuch as weeds compete with crops for light, water and soil nutrients. Reduction of herbicidal injury to crops without an unacceptable corresponding reduction of herbicidal action on the weeds can be accomplished by use of crop protectants known as herbicide "antidotes" or "safeners".
There are several known highly-phytotoxic acetanilide herbicides which provide excellent preemergent control of a broad spectrum of grassy and broadleaf weeds. Examples of three such acetanilides are acetochlor, 2-chloro-2'-methyl-6'-methoxy-N-(isopropoxymethyl) acetanilide, and 2-chloro-2'-methyl-6'-trifluormethyl-N-(ethoxymethyl) acetanilide. Such acetanilide herbicides provide highly effective control of grassy and broadleaf weeds typically found in corn or sorghum crops, but at the cost of unacceptably high injury to the corn or sorghum crops. Because of excessive crop injury, general commercial use of these acetanilide herbicides in corn and grain sorghum has not been feasible, especially in soils containing less than about six percent organic matter.
There are several classes of quinoline-type compounds known as antidotes for herbicides. U.S. Pat. No. 4,033,756 to Hoffmann describes the compound N-dichloroacetyl-1,2,3,4-tetrahydroisoquinoline as an antidote for protecting the seed of corn, grain sorghum and rice, from injury by thiocarbamate herbicides, such as triallate, or acetamide herbicides such as alachlor. Bayer West German Application No. 28 28 222 describes this same compound, N-dichloroacetyl-1,2,3,4,-tetrahydroisoquinoline, as an antidote for protecting corn from a surface-applied pre-emergent treatment of a metazachlor-type herbicide. Bayer South African Pat. No. 79/3212 and counterpart Bayer Canadian Pat. No. 1,114,824 describe the compound N-dichloroacetyl-1,2,3,4-tetrahydroquinaldine as an antidote for protecting corn from a surface-applied pre-emergent treatment of a metazachlor-type herbicide. Bayer West German Application No. 29 30 450 describes the compound N-(.alpha.-chloropropionyl)-1,2,3,4-tetrahydroisoquinoline as an antidote for protecting corn from a surface-applied pre-emergent treatment of a metazachlor-type herbicide.
U.S. Pat. No. 4,137,070 to Pallos et al describes a large class of antidote compounds including the compound dichloroacetyl-2-methyldecahydroquinoline.
None of these publications mentions the three highly-phytoxic acetanilides acetochlor, 2-chloro-2'-methyl-6'-methoxy-N-(isopropoxymethyl)acetanilide, or 2-chloro-2'-methyl-6'-trifluoromethyl-N-(ethoxymethyl)acetanilide.
Weed control for crops, especially corn crops, is one of the oldest and most highly developed areas in weed science. For a herbicide product to be accepted commercially for corn crops, such herbicide product must provide a relatively high level of control of both grassy and broadleaf weeds in corn, in addition to meeting several other criteria. For example, the herbicide must possess relatively high unit activity so that lower rates of herbicide application are feasible. Lower application rates are desirable in order to minimize exposure of the environment to the herbicide. At the same time, such herbicide must be selective in herbicidal effect so as not to injure corn crops. Herbicidal selectivity can be enhanced by use of an appropriate antidote in combination with the herbicide. But identification of an antidote having high safening activity suitable for a commercially-effective herbicide is a highly complicated task. Whether a compound or class of compounds provides efficacious antidote or safening activity is not a theoretical determination, but must be done empirically. Safening activity is determined empirically by observing the complex interaction of several biological and chemical factors, namely: the type of herbicide compound; the type of weed to be controlled; the type of crop to be protected from weed competition and herbicidal injury; the antidote compound itself; and the relative rates of application of the herbicide and antidote compounds required for effective weed control with low crop injury. Moreover, the herbicide and antidote must each possess chemical and physical properties enabling preparation of a stable formulation which is environmentally safe and easy to apply to the field.